Thunder and lightning in Ireland
Why Ireland gets so few thunderstorms, the two kinds it does get, where they strike most, the hail and flash floods that come with them, and how to stay safe when the sky turns.
Thunder is rare enough in Ireland that people stop what they are doing to listen to it. Most of the country hears it on between five and ten days a year, against thirty or more in parts of central Europe, and a whole summer can pass without a storm. But when they come they can be fierce, and the floods they bring are among the costliest weather events of the year.
Why so few?
A thunderstorm needs warm, moist air near the ground and much colder air above it, so that the warm air rises hard and fast enough to build a towering cloud with ice in its top. Ireland rarely has the warm half of the recipe. The Atlantic keeps summer afternoons in the high teens or low twenties, and the sea breeze cuts the land's heating along the coasts. The result is that the deep, boiling clouds of a continental summer rarely form here, and our storms are smaller, shorter and less electric.
Summer storms: heat and the Spanish plume
The thunder most people know comes on a warm, humid afternoon in June, July or August, after a morning of hazy sunshine. The land heats, the air rises, and by three o'clock the fair-weather cumulus has grown into a dark tower with an anvil top. These storms favour the inland counties and the east, where the sun has most effect: the midlands, Kildare, Carlow, Kilkenny and the lee of the Wicklow Mountains. They are slow-moving, which is why a single one can drop 30 mm of rain on a parish in an hour while the next town stays dry.
The biggest outbreaks come with what forecasters call a Spanish plume: a tongue of very warm air drawn north from Iberia ahead of an Atlantic front, bringing a sticky night, temperatures in the high twenties and then a line of storms that crosses the country from the south on a summer evening. These are the days of the orange thunderstorm warning, the lightning show over Dublin at midnight and the flooded underpasses the next morning.
Winter thunder: the cold showers
The other kind of thunder comes with the Atlantic showers of autumn and winter, when the air aloft is very cold and the sea is still relatively warm. The showers that stream in on a north-westerly behind a depression can turn thundery, usually with a burst of hail, a squall of wind and a single flash and rumble. The west and north coasts, from Kerry to Donegal, get most of these, often in November or at the back end of a storm in January. They are brief, and the radar shows them as small, intense blobs racing in off the sea; the guide to reading the radar explains how to track them.
Hail and flash floods
Hail is thunder's companion, and Ireland gets it in both seasons: soft, small hail in winter showers, and the larger, harder stones of a summer storm, which have dented cars and stripped crops in the midlands. The greater danger is the rain. A summer storm can give a month's rainfall in an hour, far more than drains and small rivers can take, and flash flooding follows: streets in Dublin and Cork under water, a bog road in Mayo washed away, a stream in Wicklow that was a trickle at lunch running through a kitchen at four. The warnings guide explains the rainfall warning thresholds; a thunderstorm warning usually means both lightning and this kind of rain.
Where it strikes most
Lightning is likeliest where the storms are: the east and south-east in summer, the Atlantic coasts in winter. Mountains attract it, and so do the highest objects on flat ground: a tree in the middle of a field, a mast, a church spire. Ireland records a death or a serious injury from lightning only every few years, but livestock sheltering under a lone tree are killed most summers, and strikes on houses, with blown fuses and scorched chimneys, are commoner than people think.
Staying safe
- Count the gap. Sound travels about a kilometre every three seconds. If the thunder follows the flash within thirty seconds, the storm is within 10 km and you should be indoors or in a car. Wait thirty minutes after the last thunder before going back out.
- Get off the hill, the water and the pitch. Summits, boats, open water and open ground are the worst places to be. A golf course on a July afternoon is the classic Irish lightning setting; the Sports weather page notes thundery days in the week ahead.
- Not under a lone tree. A wood is safer than a single tree, and a dip in the ground safer than a ridge.
- Indoors, keep away from wired things. Corded phones, plumbing and anything plugged into the mains can carry a strike. A car with the windows up is safe; a tent is not.
Seeing it coming
The hourly forecast shows a thunder symbol when the models expect storms, but they are hit and miss by nature: the forecast knows the afternoon will be thundery somewhere in Leinster, not which parish. On a warm, humid day with towering clouds by lunchtime, keep an eye on the radar loop, which shows the cells forming and the direction they are moving, and treat any dark base with a sharp edge as a storm until it proves otherwise.